One method of installing a bushing into a sheet metal member, such as a hydroformed tubular member, requires piercing or blanking a hole in a portion of the sheet metal member. The sheet metal member is then passed on to a secondary operation where the edge portion surrounding the pierced or blanked hole is extruded or drawformed to form an annular flange portion. A steel bushing is forced into the opening so as to form an interference fit between the annular flange portion and the bushing. A tight tolerance is required between the extruded drawformed annular flange portion and the bushing to provide the appropriate interference fit.
FIGS. 1A-3B illustrate conventional methods of installing a bushing in a hydroformed motor vehicle frame member. FIGS. 1A and 1B illustrate a punch 11 fastened to an upper moving ram of a reciprocating press, and an extrusion die 12 fixed to a lower stationary press bed. A tubular metal frame member which has been hydroformed from a seam-welded piece of sheet metal is partially shown and indicated at 100.
The extrusion die 12 is inserted in an open end of the hydroformed tubular member 100 and then lifted upwards to support the underside of an upper wall 10 of the member 100. The press ram then begins its downward stroke to force the punch 11 into contact with the metal material of the upper wall 10, as shown in FIG. 1A. When the press tonnage overcomes the tensile strength of the metal wall 10, the punch 11 shears the metal wall 10 against an opening 13 in die 12. The die opening 13 is defined at its upper end by a "hard" or 90.degree. corner 35.
The scrap material or slug 15 passes through the die opening 13 and is eventually discarded, leaving hole 14 in the upper wall 10 of the tubular member 100. The diameter of the hole 14 corresponds to the outer diameter of punch 11. As shown in FIG. 1B, the press ram then starts its upward movement back to its starting position to move punch 11 away from the tubular member 100, and the tubular member 100 is then removed from the extrusion die 12. As a result of this hole forming operation, the edge portion surrounding the hole 14 will have a sharp, burred corner.
As shown in FIGS. 2A and 2B, after hole 14 is formed, the tubular member 100 is then moved to a drawforming station. At the drawforming station, a second die 20 is positioned inside the tubular member 100 and a press ram with a drawforming punch 16 is then cycled through a downstroke to locate a punch pilot 17 in alignment with the hole 14. The press ram then continues its downstroke to force a forming portion of the punch into engagement with the inner edge portion surrounding the hole 14. As shown in FIG. 2A, the press ram lowers the punch 16 lowers to a preset depth, so that the forming portion thereof draws the inner edge portion downwardly against a beveled upper edge 36 of the second die 20, thereby forming an annular drawformed flange portion 18. The press then begins its upstroke and removes upper punch 16 and punch pilot 17 from the tubular member 100. This causes the sharp, burred corner of annular flange portion 18 to be burnished smooth. As indicated by the arrow in FIG. 2B, the die 20 is then pulled out from the open end of the tubular frame member 100.
FIGS. 3A and 3B illustrate a steel can bushing 23 which has been preassembled in a separate operation from the punching operation. The tubular member 100 is placed in a holding fixture (not shown) that supports the underside of upper wall 10 close to the inner edge 25 of drawn flange 18. The bushing 23 is located directly above the hole 14 either manually or via the use of a machine. Downward pressure is applied to top surface 26 of the bushing 23 to force the busing through the hole 14 in the direction indicated by arrow 24. The pressure is then released and the finished assembly shown in FIG. 3B is produced with the bushing 23 being retained through an interference fit relation with the annular drawformed flange portion 18.
The problem with the above-described conventional method is that the fit between the exterior surface of the steel can bushing 23 and the annular drawformed flange portion 18 must be kept tight to ensure that the bushing 23 does not become separated from the tubular member 100. However, the fit must not be so tight that an extraordinarily high amount of effort is required to force the bushing 23 into the hole 14. Also, removal of the drawforming punch 16 burnishes the inner corner of the flange portion 18 which engages the bushing 23. As a result of this burnishing, the corner is smoother and does not "bite" into the bushing exterior to resist removal of the bushing.